Method and device for serial data transmission which is adapted to memory sizes

ABSTRACT

A method is described for serial data transmission in a bus system having at least two participating data processing units, the data processing units exchanging messages via the bus, the sent messages having a logical structure in accordance with CAN standard ISO 11898-1. When a first changeover condition is present, then, deviating from CAN, the data field of the messages can include more than eight bytes, the values of the data length code being interpreted, given the presence of the first changeover condition to determine the size of the data field. For forwarding data between the data field and the application software, at least one buffer memory is provided, and, if the size of the data field differs from the size of the buffer memory used, the forwarded quantity of data is adapted at least corresponding to the difference in size between the data field and the buffer memory.

FIELD

The present invention relates to a method and to a device for serialdata transmission, adapted to memory sizes, between at least twoparticipants in a serial bus system.

BACKGROUND INFORMATION

The standards of the family ISO 11898-1 through -5 describe theController Area Network (CAN), as well as an expansion of CAN referredto as Time-Triggered CAN (TTCAN). The standards are also referred tobelow as the CAN standard. The media access control method used in CANis based on a bitwise arbitration. In the bitwise arbitration, aplurality of participating stations can simultaneously transmit data viathe channel of the bus system without thereby disturbing the datatransmission. In addition, during the transmission of a bit via thechannel the participating stations can determine the logical state (0or 1) of the channel. If a value of the sent bit does not correspond tothe determined logical state of the channel, the participating stationthen terminates access to the channel. In CAN, the bitwise arbitrationis standardly carried out on the basis of an identifier contained in amessage that is to be transmitted via the channel. After a participatingstation has completely transmitted the identifier on the channel, itknows that it has exclusive access to the channel. Thus, the end of thetransmission of the identifier corresponds to a beginning of a clearanceinterval within which the participating station can use the channelexclusively. According to the protocol specification of CAN, otherparticipating stations may not access the channel, i.e., transmit dataon the channel, until the transmitting participating station hastransmitted a checksum field (CRC field) of the message. Thus, an endtime point of the transmission of the CRC field corresponds to an end ofthe clearance interval.

Thus, through the bitwise arbitration a disturbance-free transmissionvia the channel is achieved of the message that has obtained thearbitration method. The protocols of CAN are suitable in particular forthe transmission of short messages under real-time conditions, suchthat, through suitable assignment of the identifier, it can be ensuredthat particularly important messages almost always obtain thearbitration and are successfully transmitted.

With the increasing networking of modern vehicles and the introductionof additional systems for improving, for example, driver safety ordriver comfort, the demands become greater on the quantities of datathat are to be transmitted and on the latency times that are permissibleduring transmission. Examples include driving dynamics regulatingsystems, such as the electronic stability program ESP, driver assistancesystems, such as automatic distance regulation ACC, or driverinformation systems such as traffic sign recognition (see, for example,the descriptions in “Bosch Kraftfahrtechnisches Handbuch,” 27^(th) ed.,2011, Vieweg+Tuebner).

German Patent Application No. DE 103 11 395 A1 describes a system inwhich asynchronous serial communication can alternatively take place viaan asymmetrical physical CAN protocol or via the symmetrical physicalCAN protocol, and in this way a higher data transmission rate or datatransmission security is achievable for the asynchronous communication.

German Patent Application No. DE 10 2007 051 657 A1 describes the use,in the exclusive time windows of the TTCAN protocol, of an asynchronous,fast, non-CAN-conforming data transmission in order to increase thetransmitted data quantity.

In “Overclocking of controller area networks” (Electronics Letters, vol.35, no. 22 (1999), page 1924, G. Cena and A. Valenzano address theeffects of an overclocking of the bus frequency in sub-regions of themessages on the effectively achieved data rate.

SUMMARY

Below, the present invention and some of its advantages are described onthe basis of figures and exemplary embodiments. The subject matter ofthe present invention is not limited to the presented and describedexemplary embodiments.

In accordance with the present invention, messages having a logicalstructure according to CAN standard ISO 11898-1 are transmitted in a bussystem having at least two participating data processing units, thelogical structure including a start-of-frame bit, an arbitration field,a control field, a data field, a CRC field, an acknowledge field, and anend-of-frame sequence, and the control field including a data lengthcode that contains an item of information about the length of the datafield.

In accordance with the present invention, an example method is providedthat is distinguished in that, deviating from CAN standard ISO 11898-1,the data field of messages can include more than eight bytes, the valueof the four bits of the data length code being interpreted in a mannerdeviating at least partly from CAN standard ISO 11898-1 in order todetermine the size of the data field, and that for the forwarding ofdata between the data field and the application software, at least onebuffer memory is provided for use, and, if the size of the data fielddiffers from the size of the buffer memory used, the forwarded quantityof data is adapted at least corresponding to the size difference betweenthe data field and the buffer memory used. In this way, the advantage isachieved that the application software can continue to be used inunmodified fashion, and the size of the communication controller neednot be unnecessarily increased, even if the size of the data field maybe increased in comparison with the CAN standard.

Advantageously, from the data field of a message received via the bus, adata quantity corresponding to the size of the buffer memory used, inparticular including eight bytes, is selected according to a specifiedor specifiable selection method, and is forwarded to the buffer memoryif the size of the data field is larger than the size of the buffermemory used, typically eight bytes. In the data field of a message thatis to be sent via the bus, the content of the buffer memory is enteredinto at least one specified or specifiable region of the data field ofthe message, and the remaining region or regions of the data field arefilled according to a specified or specifiable method if the size of thedata field is greater than the size of the buffer memory used. So thatthe message length does not increase unnecessarily, it is advantageousto fill the bits into the filled regions of the data field of a messageto be sent via the bus in such a way that, according to the rules of CANstandard ISO 11898-1, no stuff bits need be inserted in these regions.

Through the setting up of an unambiguous allocation between the contentof the data length code and the length of the data field, a high degreeof flexibility is advantageously achieved with regard to therepresentable size of the data field.

In an advantageous embodiment of the example method, the enlargement ofthe data field and the adaptation of the interpretation of the contentof the data length code take place as a function of a first changeovercondition, so that when the first changeover condition is present themethod according to the present invention is used, while otherwise thedata transmission takes place in accordance with the normal CANstandard. Messages according to the example embodiment of the presentinvention can be distinguished from messages according to the CANstandard via an identifier in the arbitration field and/or in thecontrol field. The identifier is evaluated in the participating dataprocessing units in order to determine the first changeover condition,so that the receive process is adapted to the size of the data field asa function of the first changeover condition. In this way, an advantageis achieved that example devices according to the present invention canbe used both in standard CAN bus systems and in new bus systemsaccording to the present invention having potentially larger datafields.

In addition, it is possible, in the case in which the data field isenlarged in accordance with the present invention, to use a modifiedpolynomial to calculate the checksum and to transmit it in the CRCfield. This has the advantage that the reliability of the errorrecognition is maintained even for larger transmitted data quantities.In a particularly advantageous specific embodiment, at the beginning ofa message a plurality of calculations of checksums are started inparallel, and, as a function of the presence of a (possibly the same)changeover condition, and/or of the content of the data length code, adecision is made as to which result of one of these calculations isused, or is transmitted in the CRC field. In this way, it is possible tosend the information concerning whether a message is transmittedaccording to the method in accordance with the standard or in accordancewith the modified method according to the present invention along withthe message without informing the receiver ahead of time as to themethod used. The checksums for checking the correct data transmissionare present for both methods, and can be evaluated as needed.

If the example method is combined with a changeover of the bit lengthfor example for the bits of the data field and of the CRC field, thefurther advantage is then achieved that a larger quantity of data can betransmitted in accelerated fashion, and the average data transmissionrate of the bus system is increased. Here as well it may be advantageousto link the changeover to a changeover condition, and to provide themessages transmitted in modified fashion, having modified bit length,with a corresponding identifier.

The changeover conditions present in each case are communicated to thereceivers by one or more identifiers. Here it is particularlyadvantageous if at least one of the identifiers takes place through afirst identifier bit whose position is situated between the last bit ofthe identifier and the first bit of the data length code, and at whoseposition there is situated, in messages according to CAN standard ISO11898-1, a bit having a defined value, in order to make it possible touse devices according to the present invention both in standard CAN bussystems and also in new bus systems according to the present invention.

The further identification via a further identifier bit (BRS)advantageously takes place via a bit situated between the firstidentifier bit and the first bit of the data length code.

In this way, the changeover of the bit length can take place independentof the changeover of the CRC calculation, or of the size of the datafield, and it is possible to react in a flexible manner to particularconditions of the bus system.

For the understanding of the transmission method used in each case, itis advantageous to provide one or more additional status bits throughwhich information relating to the data transmission method deviatingfrom CAN standard ISO 11898-1 can be provided for the applicationsoftware. For example, it would be possible to provide status bits inorder to communicate the successful transmission, successful reception,or the type of error that most recently occurred. As a function of thefrequency of the occurrence of errors in a data transmission methoddeviating from CAN standard ISO 11898-1, it is advantageously possibleto switch back to the transmission method according to CAN standard ISO11898-1 and to signal this using a further status bit.

In the message memory and/or buffer memory, one or more additionalmessage bits can advantageously be provided that identify the methodthat is used, or that is to be used, for data transmission for therespective message. Here, for example the identifiers provided in themessage can be entered.

The example method can advantageously be used in the normal operation ofa motor vehicle for the transmission of data between at least twocontrol devices of the motor vehicle connected via a suitable data bus.However, the example method is equally advantageously usable duringmanufacture or maintenance of a motor vehicle for the transmission ofdata between a programming unit connected to a suitable data bus for thepurpose of programming and at least one control device of the motorvehicle connected to the data bus. The example method is likewiseadvantageously usable in industrial applications if larger quantities ofdata have to be transmitted, for example for control purposes. Inparticular when, due to the length of the transmission path duringarbitration, a reduced data rate must be used so that all participantswill have the possibility of obtaining access to the bus, the method canprovide a higher data transmission rate, in particular in combinationwith the changeover of the length of the data field and the reduction ofthe bit length.

A further advantage is that a CAN standard controller need be modifiedonly minimally in order to be able to operate in accordance with thepresent invention. A communication controller according to the presentinvention that can also operate as a CAN standard controller is onlynegligibly larger than a conventional CAN standard controller. Theassociated application program does not have to be modified, andadvantages in the speed of the data transmission are already achieved.

Advantageously, significant parts of the CAN conformance test (ISO16845) can be taken over. In an advantageous realization, thetransmission method according to the present invention can be combinedwith supplementations of the TTCAN (ISO 11898-4).

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention is explained in further detailbelow on the basis of the figures.

FIG. 1 a shows two alternatives (standard/extended) for the structure ofmessages in the CAN format according to CAN standard ISO 11898-1.

FIG. 1 b shows two analogous alternatives for the format of themessages, modified in comparison thereto according to the presentinvention, in accordance with an exemplary embodiment of the presentinvention.

FIG. 2 shows various possibilities for interpreting the content of thedata length code deviating in accordance with the present invention fromCAN standard ISO 11898-1.

FIG. 3 schematically shows the receive process in accordance with thepresent invention at a participating station of the bus system accordingto an exemplary embodiment of the present invention.

FIG. 4 schematically shows the receive process in accordance with thepresent invention at a participating station of the bus system accordingto a further exemplary embodiment of the present invention.

FIG. 5 shows, for an exemplary embodiment of the present invention, theformat of messages modified in accordance with the present invention, inwhich a different bit length is used additionally in defined regionsinside the message.

FIG. 6 shows an example of the adaptation in accordance with the presentinvention of the data quantity forwarded between the data field and theapplication software.

FIG. 7 shows two alternatives (standard/extended) for the format of themessages modified in accordance with the present invention according toa further exemplary embodiment of the present invention, in whichseparate bits are used for the changeover of data field size and bitlength, and additional bits are also incorporated into the controlfield.

FIG. 8 shows the receive process adapted in accordance with theexemplary embodiment of the present invention, running on aparticipating station of the bus system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 a shows the structure of messages as used on a CAN bus for datatransmission. The two different formats, standard and extended, areshown. The example method according to the present invention can beapplied to both formats in suitable specific embodiments.

The message begins with a start-of-frame (SOF) bit, which signals thebeginning of the message. There follows a segment whose main purpose isto identify the message, and on the basis of which the participants ofthe bus system decide whether they will receive the message or not. Thissegment is referred to as the arbitration field, and contains theidentifier. There follows a control field containing, inter alfa, thedata length code. The data length code contains information about thesize of the data field of the message. There then follows the actualdata field, which contains the data that is to be exchanged between theparticipants of the bus system. There follows the CRC field, having thechecksum, which has 15 bits, and having a delimiter, followed by twoacknowledge (ACK) bits, whose purpose is to signal the successfulreception of a message to the sender. The message is terminated by anend-of-frame (EOF) sequence.

In the CAN transmission method according to the standard, the data fieldmay contain a maximum of eight bytes, i.e., 64 bits of data. Accordingto the standard, the data length code includes four bits, and cantherefore assume 16 different values. From this range of values, intoday's bus systems only eight different values are used for thedifferent sizes of the data field, from one byte up to eight bytes. Adata field of zero bytes is not recommended in the CAN standard, andsizes above eight bytes are not allowed. The assignment of the values ofthe data length code to the sizes of the data field is shown in FIG. 2,in the column “CAN standard.”

FIG. 1 b shows, in an analogous representation and in comparison, themodified messages that are to be transmitted in accordance with anexample embodiment of the present invention, derived in each case fromthe two standard formats.

In the transmission method modified according to an example embodimentof the present invention, the data field may also contain more thaneight bytes; namely, in the version shown it may contain up to K bytes.Differing from the CAN standard, additional values that the data lengthcode can assume are used to identify larger data fields. For example,the four bits of the data length code can be used to represent thevalues from 0 to 15 bytes. However, other allocations can also be made;for example, it is possible to use the data length code DLC=0b0000,which is not standardly used in today's CAN messages, for a furtherpossible size of the data field, for example for the size 16 bytes.

These two possibilities are shown in FIG. 2 in table form, as DLC 1 andDLC 2. In these cases, the maximum size of data field K has the value 15or 16. A further possibility is that, for the values of the data lengthcode larger than 0b1000 and up to 0b1111, the associated sizes of thedata field are increased by a larger increment. An example of this caseis shown in the table as DLC 3. In this variant, the maximum size ofdata field K reaches the value 64 bytes. Of course, a different choiceis also possible, for example an increment of four bytes in each case.

In order to ensure that a communication controller according to thepresent invention can determine the way in which it has to interpret thecontents of the data length code, it is advantageous for it toautomatically recognize whether the communication of the bus system istaking place according to the CAN standard or according to the examplemethod of the present invention. A possibility for this is to use areserved bit inside the arbitration field or inside the control fieldfor identification, so that from this identifier the communicationcontroller can derive a first changeover condition, as a function ofwhich it selects the transmission method. For example, the second bit ofthe control field, designated r0 in FIG. 1 b, can be used for theidentification.

The determination can also be chosen as a function of the identifierformat. For standard addressing, in this way a possibility foridentifying the messages according to the present invention is theinsertion of a recessive EDL (extended data length) bit into the controlfield at the position of the r0 bit, which is always dominant in the CANstandard. For extended addressing, the recessive EDL bit in the controlfield can occur at the position of the r1 bit, which is always dominantin the CAN standard.

Another possibility is to use the SRR bit, which in the CAN standardalways has to be sent in recessive fashion, but which is also acceptedin dominant fashion by the bus participants receiving the message. Bitcombinations can also be evaluated for the determination of the firstchangeover condition.

A further possibility would be to specify the use of the extended formatfor the transmission method modified according to the present invention.Messages in the extended format are recognized by the bus participantsvia the value of the IDE bit (cf. FIG. 1 a), and this bit cansimultaneously represent the first changeover condition, so that themodified transmission method is always used for extended messages.Alternatively, it would also be possible in extended messages to use thereserved bit r1 for the identification or for the derivation of thefirst changeover condition. However, the reserved bit can also, asexplained below, be used to derive a second changeover condition forchanging over between more than two different sizes of the data field orallocations between values of the data length code and data field sizes.

Alternatively, however, it is also possible to apply the example methodin communication controllers that are suitable for that purpose but arenot also designed for communication according to the CAN standard. Inthis case, the determination of the named first changeover condition,for example as a function of a suitable identification of the messages,can be omitted. In this case, the communication controllers insteadoperate exclusively according to one of the described methods, and arecorrespondingly capable of being used only in bus systems in which onlysuch communication controllers according to the present invention arebeing used.

If, as provided in the present invention, the data field of messages isenlarged, it can make sense also to adapt the method used for the cyclicredundancy check (CRC) in order to achieve adequate security againsterrors. In particular, it may be advantageous to use a different CRCpolynomial, for example a higher-order one, and correspondingly toprovide a CRC field having a different size in the messages modifiedaccording to the present invention. In FIG. 1 b, this is shown in thatthe CRC field of the messages according to the present invention have,in the depicted example, a length of L bits, where L, deviating from theCAN standard, can be unequal, in particular can be greater than 15.

The use of a modified method for the calculation of the CRC checksum canbe signaled to the bus participants by an identifier representing athird changeover condition.

This identifier and the third changeover condition can however alsoagree with the first identifier and/or changeover condition. Here aswell, as described above, it is also possible for example to use thereserved bit r0 of FIG. 1 b for the identification, or the SRR bit canbe used. A use of the IDE bit in connection with the use of the methodin extended messages, or also of the r1 bit, is also possible.

In the CAN standard controllers, the CRC code of CAN messages that areto be sent is produced by a feedback shift register at whose input theserially transmitted bits of the message are fed in sequentially. Thewidth of the shift register corresponds to the order of the CRCpolynomial. The CRC coding takes place through a linkage of the registercontact with the CRC polynomial during the shift operations. When CANmessages are received, the serially received bits of the message arecorrespondingly shifted into the CRC shift register. The CRC test issuccessful if at the end of the CRC field all bits of the shift registerare at zero. The CRC code generation in the case of transmission and theCRC test in the case of reception both take place in hardware withoutrequiring a software intervention. A modification of the CRC codingtherefore has no effect on the application software.

In a possible specific embodiment, the communication controller isrealized in such a way that it is compatible with the CAN standard;i.e., it operates in a CAN standard bus system in a manner conforming tothe standard, while in a bus system modified according to the presentinvention, on the one hand the controller permits larger data fields inthe messages, and on the other hand it also carries out the modifiedcalculation and checking of the CRC code.

Because, at the beginning of the reception of a message, it is not yetdetermined whether a CAN message conforming to the standard, or amessage modified according to the present invention, is being received,in a communication controller according to the present invention two CRCshift registers are implemented that operate in parallel. After thereception of the CRC delimiter, if the CRC code is evaluated in thereceiver, due to the identifier according to the present invention, ordue to the third changeover condition derived for example from theidentifier or from the content of the data length code, it is alsodetermined which transmission method was used, and the shift registerassigned to this transmission method is then evaluated. As explainedabove, the third changeover condition can agree with the firstchangeover condition, which relates to the size of the data field andthe interpretation of the data length code.

At the beginning of the sending of a message, for the sender it isindeed already determined which transmission method is to be used forthe transmission. However, because it can occur that the arbitrationabout the bus access is lost and the started message is not sent, andinstead a different message is received, here as well both CRC shiftregisters are controlled in parallel.

The described implementation of two CRC shift registers operating inparallel also enables a further improvement: the CRC polynomial of theCAN standard protocol (x15+x14+x10+x8+x7+x4+x3+1) is designed for amessage length of fewer than 127 bits. If messages transmitted accordingto the present invention also use longer data fields, then in order tomaintain transmission security it makes sense to use a different, inparticular longer, CRC polynomial. The messages communicated accordingto the present invention are correspondingly provided with a modified,in particular longer, CRC field. During operation, the communicationcontrollers change dynamically between the two CRC shift registers,i.e., the shift register in accordance with the CAN standard and theshift register according to the present invention, in order to use theappropriate polynomial in each case.

It is also possible to use more than two shift registers, andcorrespondingly more than two CRC polynomials, in stepped fashion, as afunction of the length of the data field or of the desired transmissionsecurity. In this case, if compatibility with the CAN standard is to bemaintained, the corresponding identifier and the changeover conditionassociated therewith must be modified. For example, using the reservedbit r0 or the SRR bit in FIG. 1 b a first changeover condition could betriggered that identifies a changeover to longer data fields, forexample according to DLC 1 in FIG. 2, and an associated second CRCpolynomial. For messages in the extended format, a second changeovercondition could additionally be triggered, for example by reserved bitr1 or by the IDE bit in FIG. 1 b, identifying the changeover to afurther set of data field sizes, for example DLC 3 of FIG. 2, and athird CRC polynomial.

In addition, it is also possible for the first changeover condition tochange over to the possibility of longer data fields and thecorresponding interpretation of the content of the data length code, forexample via the reserved bit r0 or the SRR bit, and for thedetermination of the third changeover condition, and therewith theselection of the CRC polynomial that is to be evaluated for the CRCcheck, to then take place as a function of the content of the datalength code. The third changeover condition can correspondingly alsoassume more than two values. For example, the data field sizes could beselected in accordance with DLC 3, i.e., could assume values between 0and 64 bytes, and three CRC polynomials could then be calculated inparallel via suitable shift registers, for example the CRC standardpolynomial for data fields up to 8 bytes, a second CRC polynomial fordata fields up to 24 bytes, and a third CRC polynomial for data fieldsup to 64 bytes.

FIG. 3 shows, in a simplified representation, a segment of the receiveprocess according to an example embodiment of the present invention asit runs on a participating station of the bus system. Here, the case isshown in which compatibility with the CAN standard is achieved byadapting the behavior of the communication controller as a function ofthe first changeover condition. Although in FIG. 3 a representation hasbeen selected that is standard for the description of program sequencesin software, the method is completely suitable for hardwareimplementation.

The participant station is first in a state in which it scans the bus aslong as no communication traffic prevails on the bus. Query 302 thuswaits for a dominant bit on the bus. This bit identifies the beginningof a new message.

As soon as the beginning of a new message has been determined, in block304 the calculation begins of the at least two checksums that are to becalculated in parallel. The first checksum corresponds to the CRCcalculation of the CAN standard, while the second checksum is calculatedaccording to the new method.

Next, beginning with step 306, the further bits of the message,following the SOF bit, are received, beginning with the arbitrationfield. If a plurality of bus participants wish to send a message, herenegotiation takes place among the bus participants, according to the CANstandard method, as to which bus subscriber receives access to the bus.Depicted block 306 identifies the reception of all bits until the firstidentifier has been received, or the first changeover condition isdetermined. In the examples shown, the first changeover condition isdetermined from the arbitration field, for example from the SRR bit orfrom the IDE bit, or from the control field, for example from a reservedbit thereof (cf. FIG. 1). Subsequently, in block 308 further bits of themessage can be received until, beginning from a particular bit of themessage, the procedure becomes different as a function of the determinedfirst changeover condition. This division into different ways ofproceeding is ensured by a corresponding query or branching 310, as isshown in examples in the following.

If at branching 310, for example after the reception of the first twobits of the control field, the information is present that in accordancewith the first changeover condition the communication is taking placeaccording to the CAN standard (the path in FIG. 3 beginning with “1”),then in step 312 the further bits of the control field are read in. Fromthese bits, the data length code is evaluated according to the CANstandard, and subsequently, in step 316, the associated quantity ofdata, a maximum of 8 bytes, corresponding to the data field, isreceived. If at branching 324 the information is present that the CRCchecksum sent by the sender and the CRC checksum determined by thereceiver itself agree with one another, then in block 328 a dominantacknowledge bit is set. It is to be noted that in this case the CRCchecksum according to the standard is compared, because thecommunication is taking place according to the CAN standard. Ifagreement is not determined, then (block 330) the acknowledge bit issent recessively. Subsequently, there follow the ACK delimiter and EOFbits (see FIG. 1 b; not shown in FIG. 3).

If, in contrast, at branching 310, for example after reception of thefirst two bits of the control field, the information is present that, inaccordance with the first changeover condition, the communication methodmodified according to the present invention is to be used (the path inFIG. 3 designated with “2”), then in block 314 the further bits of thecontrol field are read in. From the result, the data length code isdetermined according to the new interpretation, of which some examplesare shown in tabular fashion in FIG. 2. In block 318, the correspondingquantity of data is received, i.e., up to 15 bytes in the case ofexample DLC 1 from the table in FIG. 2, up to 16 bytes in example DLC 2,and up to 64 bytes in example DLC 3. In block 322, the CRC field thatdeviates according to the present invention, in particular being longer,is received. If at branching 324 the information is present that the CRCchecksum sent by the sender and the CRC checksum determined by thereceiver itself agree with one another, the comparison being based inthis case on the CRC checksum deviating according to the presentinvention, then in block 328 a dominant acknowledge bit is sent.Otherwise (block 330), the acknowledgment is sent in recessive fashion.Subsequently, in step 332 or 334 respectively, there follow the ACKdelimiter and the EOF bits. With this, a receive process for a messageis terminated.

FIG. 3 shows the case in which the third changeover condition, whichdetermines the CRC to be used, agrees with the first changeovercondition relating to the size of the data field and the interpretationof the data length code. Thus, before reception 320, or 322, of the CRCchecksums it was not again queried which CRC is to be received inaccordance with the third changeover condition and to be evaluated forbranching 324. This additional query can be incorporated into thesequence through a simple modification of the flow diagram of FIG. 3, asis shown in FIG. 4.

In the receive process, modified in this way, according to FIG. 4, afterreception of the expected (according to the information from the datalength code) number of data bytes of the data field in block 316 or 318,in the query or branching 410 it is determined which value the thirdchangeover condition has. This information can, as described above, forexample have been determined from the corresponding third identifier orfrom the content of the data length code. In the depicted example, thereare three different values for the third changeover condition, namely A,B, and C. As a function of the value of the changeover condition, inblock 420, 422, and 424 a different number of bits of the CRC field arethen read in, for example 15 bits for value A, 17 bits for value B, and19 bits for value C. Subsequently, at branching 324 a check takes place,analogous to FIG. 3, of whether the CRC checksum sent by the senderagrees with the CRC checksum determined by the receiver itself, and theprocedure continues as a function thereof.

FIG. 5 again shows, for further exemplary embodiments of thetransmission method according to the present invention, the structure ofmessages in the two possible variants, the standard format and theextended format. For both variants, in FIG. 5 regions are shown in whichchangeover takes place between two states, here designated fast CANarbitration and fast CAN data. This changeover between the two statesbrings it about in this example that after termination of thearbitration for a part of the message, in particular for the data fieldand the CRC field, the bit lengths are shortened, and the individualbits are thus transmitted faster via the bus. In this way, thetransmission time for a message can be shortened compared to the methodaccording to the standard. The associated change of the temporal bitlength can for example be realized by using at least two differentscaling factors for the setting of the bus time unit relative to asmallest time unit or to the oscillator clock pulse in runningoperation. The changeover of the bit length, and the correspondingmodification of the scaling factor, are also shown as examples in FIG.5.

The transition between the states of fast CAN arbitration and fast CANdata can take place as a function of a further changeover condition thatcorresponds to an identifier of the messages that signals to theparticipants of the data transmission that the shortened bit length isbeing used. In the exemplary embodiment shown here, the selectedposition of this identifier is the reserved bit r0, transmitted beforethe data length code. It therefore corresponds to a possible position ofthe first identifier, which corresponds to the first changeovercondition and identifies the possible use of longer data fields and of amodified interpretation of the data length code, and also corresponds tothe third identifier, which corresponds to a modified CRC calculation.

Another possibility for identifying the messages according to an exampleembodiment of the present invention having shortened bit length is shownin FIG. 7. Here the messages with potentially longer data fields(associated: first identifier) and modified CRC calculation (associated:third identifier) are identified by a recessive EDL (Extended DataLength) bit, which occurs at the position of a bit transmitted indominant fashion in messages according to the CAN standard, and replacesthis bit or shifts it by one position toward the rear. For standardaddressing, the EDL bit occurs at the second position in the controlfield and shifts the r0 bit found there, which is always dominant, byone position. For the extended addressing, in the depicted example theEDL bit occurs at the first position of the control field, and replacesthe reserved r1 bit located there, which in the CAN standard is alwaystransmitted in dominant fashion. The fourth identifier, which indicatesthe use of the shortened bit length, is represented by the insertion ofan additional, recessive BRS (Bit Rate Switch) bit into the controlfield of messages according to the present invention, identified by theEDL bit. In the exemplary embodiment shown here, the position of the BRSbit is the fourth (standard addressing) or third (extended addressing)position in the control field.

The messages have the designation “CAN FD Fast.” For the two possibleaddressing variants of messages, the standard format and the extendedformat, in FIG. 7 regions are shown in which changeover takes placebetween two states, designated fast CAN arbitration and fast CAN data.This changeover between the two states brings it about, as alreadyexplained, that for the corresponding part of the message the bitlengths are shortened, and thus the individual bits are transmittedfaster via the bus. In this way, the transmission time for a message canbe shortened relative to the method according to the standard. Thetransition between the states fast CAN arbitration and fast CAN datatakes place in messages that have the first or third identifier EDL, asa function of the further identifier BRS, which signals to theparticipants in the data transmission that the shortened bit length isbeing used.

In the depicted case, in which the first identifier EDL is followed bythe second identifier BRS, in the transmission method according to thepresent invention messages are transmitted whose bit length issignificantly shortened, and whose data field size can be expanded tovalues greater than 8 bytes, and whose CRC is adapted to the larger datafield. In this way, a significant increase in the transmission capacityvia the bus system is achieved with simultaneously improved transmissionsecurity.

In the depicted example, the faster transmission begins immediatelyafter the sending of the associated identifier, and is terminatedimmediately after reaching the bit designated for the changeover back,or when a reason for starting an error frame has been recognized.

FIG. 8 shows the receive process modified in comparison with FIG. 3, inwhich in addition, as a function of the second identifier BRS, achangeover takes place between the states fast CAN arbitration and fastCAN data. If at branching 310, for example after reception of the secondbit of the control field as recessive bit EDL, the information ispresent that the communication method modified according to the presentinvention is to be used, then in block 408 the next bits of the controlfield are read in. If the bit acting as the second identifier, forexample the fourth bit BRS of the control field expanded according tothe present invention, is received with the provided value, for examplein recessive fashion, then for example at the sample point of this bitthe state fast CAN data is incorporated, i.e., a changeover takes placeto the shortened bit length (path “C”). If the relevant bit has theconverse value, i.e., in this example is dominant, then no shortening ofthe bit length takes place (path “B”). In block 412 or 414 there takesplace the reception of the remaining bits of the control field includingthe data length code and the reception of the data field according tothe size information from the data length code. In block 412 receptiontakes place with normal bit length, and in block 414 it takes place withthe shortened bit length. In block 416 or 418, the CRC field thatdeviates according to the present invention, in particular being longer,is read in. At the last bit of the CRC field, the CRC delimiter, inblock 418 changeover again takes place to the state fast CAN arbitrationwith the standard bit rate. Subsequently, at branching 324, analogous toFIG. 3, it is checked whether the CRC checksum sent by the sender agreeswith the CRC checksum determined by the receiver itself, and theprocedure continues further as a function thereof, as was already seenin FIG. 3.

The faster transmission begins for example immediately after the sendingof the associated identifier, and is terminated immediately afterreaching the bit designated for the changeover back, or when a reasonfor the start of an error frame has been recognized.

In normal operation of a motor vehicle, the method is suitable for thetransmission of data between at least two control devices of the motorvehicle connected via a suitable data bus. However, it is equallyadvantageously usable during manufacture or maintenance of a motorvehicle for the transmission of data between a programming unit,connected to a suitable data bus for the purpose of programming, and atleast one control device of the motor vehicle that is connected to thedata bus. In addition, it is also possible for the method to be used inindustrial automation, for example for the transmission of controlinformation between distributed control units that are connected to oneanother through the bus and that control the sequence of an industrialmanufacturing process. In this environment, very long bus lines can alsooccur, and it can be particularly advantageous to operate the bus systemfor the arbitration phase with a relatively long bit length, for example16, 32, or 64 microseconds, so that the bus signals can be propagatedover the entire bus system as required during the arbitration process.Subsequently, it is then possible to change over to shorter bit lengthsfor a part of the message as described so that the average transmissionrate will not become too low.

Overall, the method represents a transmission method that isdistinguished in that a CAN standard controller has to be modified onlyminimally in order to be capable of operating according to the presentinvention. A communication controller according to the present inventionthat can also operate as a CAN standard controller is onlyinsignificantly larger than a conventional CAN standard controller. Theassociated application program does not have to be modified, andadvantages are already achieved in the speed of the data transmission.Through the use of the expanded size of the data field and of theassociated DLC and CRC, the speed of the data transmission can befurther increased, and the adaptations in the application software areminimal. Large portions of the CAN conformance test (ISO 16845) can beincorporated. It is also possible to combine the transmission methodaccording to the present invention with the supplementations of theTTCAN (ISO 11898-4).

At least for particular application purposes, or in an introductoryphase, it makes sense for the data transmission method according to thepresent invention and the devices that carry it out to provide a variantthat behaves completely or largely compatibly with the applicationsoftware used. This application software can for example be theregulation software of an electronic stability program for motorvehicles, or can be the control software for an internal combustionengine of a motor vehicle. In particular for safety-critical systems,the application software is subject to extensive safeguarding programs,and it is therefore advantageous to be able to carry out theintroduction of data transmission devices modified according to thepresent invention with unmodified application software.

The forwarding of data between the data field of a message and theapplication software takes place in communication controllers via abuffer memory provided for this use, which in the CAN standard has aspecified size of, for example, eight bytes, and an associated messagememory device. In the context of the present invention, the buffermemory and message memory can also refer to a region within a largermemory unit allocated for the corresponding use.

Correspondingly, in systems that carry out a data transmission accordingto the CAN standard, the application software exchanges data packets ofat most eight bytes in size with the data field of CAN messages, via, inmost cases, numerous message memory devices and buffer memory devices.If the application software remains unmodified, then the message memoryand buffer memory can also be left at the specified size of for exampleeight bytes in order to avoid unnecessarily increasing the size, or chipsurface, of the communication controller according to the presentinvention. If the size of the data field of a message according to thepresent invention (for example 16 bytes) differs from the size of thebuffer memory used (for example eight bytes), it is necessary to adaptthe protocol control unit of the communication controller correspondingto the difference in size between the data field and the buffer memory.

A further exemplary embodiment of the data transmission method of thepresent invention, illustrated in FIG. 6 for the example of a message instandard format having a data field 16 bytes in size, therefore providesthat, with regard to the serial transmission of bits via the bus, thefull functionality of the method is executed; however, only eight bytesof relevant useful data are transmitted in the data field. Therefore,while, given the corresponding presence of a changeover condition, forexample a message having a data field 16 bytes in size and an associatedidentifier are transmitted via the bus, and the message is checked forcorrect transmission via a correspondingly adapted CRC polynomial, onlyeight bytes of useful data are handed over to application software 640via buffer memory 620 and, for example, a message memory 630 in thecommunication controller according to the present invention. In the caseof transmission, application software 640 writes eight bytes of usefuldata into buffer memory 620, for example via a further message memory630 or the same message memory 630, these useful data being enteredaccording to a specified or specifiable method into the data field,which is for example 16 bytes in size, of the message to be sent. Ofcourse, it is also possible to limit the method to a size of the buffermemory differing from eight bytes, for example six or four bytes. Thismakes sense above all if the application software also provides thislimitation. The selection of the size of the data field of 16 bytes isto be understood only as an example, and can for example also includevalues of 24, 32, or 64 bytes. Numerous other possibilities can belearned for example from FIG. 2. There is a relation between thequantity of data provided from the message to the application software,or provided by the application software for entry into the data field ofthe message, and the size of buffer memory 620.

In this exemplary embodiment, as symbolized in FIG. 6 by double arrow600, during the serialization of the data that are to be transmitted theprotocol control unit of the communication controller places for examplethe eight useful data bytes from buffer memory 620 into the first eightserial bytes of the data field, and, as shown in FIG. 6 by arrow 610,fills the further bits of the data field with specified, specifiable, orarbitrary filling data, for example with a particular bit pattern. Itmakes sense that here a bit sequence is used that does not result in aninsertion of additional stuff bits according to the rules of CANstandard ISO 11898-1, because otherwise the data field would be enlargedunnecessarily. Thus, for example for each unused byte of the data field,a bit sequence 0b00110011 or 0b11001100 can be inserted.

It is also possible for the for example eight useful data bytes ofbuffer memory 620 to be entered not into the first eight bytes of thedata field but rather at a different position, or in a plurality ofdifferent regions within the data field. The position of the useful databytes need only be uniformly specified or specifiable for the busparticipants.

The receiving bus participants read the message according to the methodof the present invention, for example carry out the CRC test, andconfirm the correct reception via acknowledge. The protocol control unitin the communication controller of the receiving bus participantconverts the serially received bits of the message and extracts from thefor example 16 bytes of the data field the eight bytes of useful datawritten to buffer memory 620. This is also symbolized in FIG. 6 bydouble arrow 600. The remaining filling data are discarded. Thus, onlythe eight bytes of useful data are handed over to application software640 via buffer memory 620 and for example message memory 630.Application software 640 itself does not determine any difference incomparison with the use of the transmission method according to thestandard.

Further supplementations are useful for the error-proof implementationof the various specific embodiments of the data transmission methodaccording to the present invention, in particular the method havingshortened bit length in sub-regions of the message, having modifiedlength of the data field and modified interpretation, possibly inmultiple stages, of the content of the data length code, with the use ofdifferent CRC checksums, or also with adaptation of the forwardedquantity of data to the difference in size between the data field andbuffer memory.

Through suitable status registers, for example in the CAN standard it isensured that the reaching of an error status (error warning, errorpassive, bus off) that occurs when there is a corresponding accumulationof errors in the data transmission can be read off for example by themicrocontroller or the application software. In addition, successfultransmit and receive processes are signaled by flags (TxOK, RxOK), andan item of information is kept ready concerning the respectively lastbus event (LEC, Last Error Code). Some or all of this information can beheld ready in multiple fashion in communication controllers according tothe present invention, in storage regions provided for this purpose, sothat for example through the associated microcontroller or theapplication software running thereon the respective information can beacquired separately and stored in accordance with the data transmissionmethod currently being executed. In this way, it can be determinedwhether, given a transmission method that is modified relative to theCAN standard, for example having a longer data field or a shorter bitlength in regions of the message, a particular error pattern occurs moreoften than is the case in the CAN standard transmission method.

An additional error status can be introduced and signaled by a statusbit that, when there is an accumulation of errors in the modifiedtransmission mode, switches the data transmission permanently back tothe CAN standard transmission method. The items of information TxOK,RxOK, and LEC can be acquired separately for the various datatransmission methods. Alternatively, for the LEC the information canadditionally be stored indicating in which transmission mode the erroroccurred.

In addition, it can be useful to determine, for received messages, themethod with which they were received and with which of the possibleidentifiers they were received, and, for messages that are to be sent,to determine individually according to which method they are to be sentand with which identifier they are to be sent. For this purpose, some orall receive message memory units and/or buffer memory units can besupplemented with a number of additional bits that correspond to thepossible identifiers. It is also possible, in further bits provided forthis purpose, to store the information separately concerning in whichstate, or under which changeover condition, the respective message wasreceived. Analogously, some or all transmit message storage devicesand/or buffer storage devices can be provided with additional bits inwhich it is entered that the respective message is to be sent using amodified method and/or with a corresponding identifier.

Finally, it can be useful to provide a communication controlleraccording to the present invention in such a way that through a suitablereconfiguration it can be switched between the applicationsoftware-compatible mode, with a limitation of the data quantityforwarded between the data field of the message and the applicationsoftware to for example 8 bytes, and a data transmission-optimized mode,with use of the full size of the data field using a buffer memorydesigned to be correspondingly large. In this case, it is true that thecorresponding savings in size or chip surface cannot be achieved throughthe use of a correspondingly smaller buffer memory and/or messagememory, but the communication controller is capable of being used veryflexibly both in bus participants that continue to use an existingapplication software and also in bus participants for which a new, datatransmission-optimized software was created.

This communication controller capable of being switched over should thenalso make visible the mode currently being used, i.e., the applicationsoftware-compatible mode or the data transmission-optimized mode, in astorage region provided for this purpose, for example using acorresponding status bit.

In the above description of the present invention, references to ISOstandards are to be understood as referring to the version of thecorresponding ISO standard valid at the time of the present application.

1-28. (canceled)
 29. A method for serial data transmission in a bussystem having at least two participating data processing units thatexchange messages via the bus, sent messages having a logical structurein accordance with CAN standard ISO 11898-1, the logical structureincluding a start-of-frame bit, an arbitration field, a control field, adata field, a CRC field, an acknowledge field, and an end-of-framesequence, the control field including a data length code that containsan item of information about the length of the data field, wherein giventhe presence of a first changeover condition, deviating from CANstandard ISO 11898-1, the data field of the messages can include morethan eight bytes, at least one buffer memory being provided forforwarding of data between the data field and application software, themethod comprising: interpreting values of the data length code, at leastpartly in a manner deviating from CAN standard ISO 11898-1, to determinea size of the data field, when the first changeover condition ispresent; and if the size of the data field differs from the size of thebuffer memory, adapting the data to be forwarded to the applicationsoftware in a manner at least corresponding to the difference in sizebetween the data field and the buffer memory.
 30. The method as recitedin claim 29, further comprising: if the size of the data field isgreater than the size of the buffer memory used, selecting from the datafield of a message received via the bus a quantity of data correspondingto the size of the buffer memory according to a specified selectionmethod, and forwarding the quantity of data to the buffer memory. 31.The method as recited in claim 29, further comprising: entering contentsof the buffer memory into at least one region of the data field of amessage to be sent via the bus, and filling remaining regions of thedata field according to a specified method if the size of the data fieldis greater than the size of the buffer memory used.
 32. The method asrecited in claim 29, wherein if the size of the data field of a messagereceived via the bus is more than eight bytes, eight specified bytes ofthe data field are given to the application software via the at leastone buffer memory.
 33. The method as recited in claim 32, wherein if thesize of the data field of a message to be sent via the bus is more thaneight bytes, remaining bits of the data field are filled with specifiedvalues.
 34. The method as recited in claim 33, wherein the bits infilled regions of the data field of a message to be sent via the bus arefilled in such a way that, in accordance with the rules of CAN standardISO 11898-1, no stuff bits need be inserted in these regions.
 35. Themethod as recited in claim 34, wherein as a function of the value of thefirst changeover condition, each of possible value combinations of thefour bits of the data length code is assigned to one of the allowablesizes of the data field.
 36. The method as recited in claim 25, whereinthe messages in which the data field includes more than eight bytes andthe values of the data length code are interpreted in a manner deviatingat least partly from CAN standard ISO 11898-1 to determine the size ofthe data field, are distinguished from messages conforming to the CANstandard through a first identifier in at least one of the arbitrationfield and the control field.
 37. The method as recited in claim 36,wherein the first identifier is evaluated in the participating dataprocessing units to determine the first changeover condition, so that areceive process is adapted to the size of the data field as a functionof the first changeover condition.
 38. The method as recited in claim37, wherein a first identification takes place through a firstidentifier bit whose position is situated between the last bit of thefirst identifier and the first bit of the data length code, and at whoseposition there is situated, in messages according to CAN standard ISO11898-1, a bit having a defined value.
 39. The method as recited inclaim 29, wherein as a function of the value of a changeover condition,the CRC field of messages can have at least two different numbers ofbits, at least one of the valid numbers of bits in the CRC field being anumber of bits deviating from CAN standard ISO 11898-1, a generatorpolynomial deviating from CAN standard ISO 11898-1 being used todetermine contents of a CRC field having a deviating number of bits. 40.The method as recited in claim 38, wherein, as a function of the valueof a further changeover condition, a temporal bit length inside amessage can assume at least two different values, the temporal bitlength being, for at least one first specifiable region inside themessage, greater than or equal to a specified minimum value ofapproximately one microsecond, and the temporal bit length having, in atleast one second specifiable region inside the message, a value that isreduced in comparison with the first region.
 41. The method as recitedin claim 40, wherein the at least two different values of the temporalbit length within a message are realized through the use of at least twodifferent scaling factors for setting a bus time unit relative to oneof: i) a smallest time unit, or ii) an oscillator clock pulse in runningoperation.
 42. The method as recited in claim 40, wherein the messagesin which, as a function of the value of a further changeover condition,the temporal bit length inside a message can assume at least twodifferent values, can be recognized via a further identifier in at leastone of the arbitration field and in the control field.
 43. The method asrecited in claim 42, wherein the value of the further changeovercondition in the participating data processing units one of: i) isdetermined as a function of the further identifier, ii) agrees with thefirst changeover condition, or iii) is derived from the first changeovercondition, the receive process being adapted to the different values ofthe bit length inside a message as a function of the value of thefurther changeover condition.
 44. The method as recited in claim 43,wherein a further identifier takes place through a further identifierbit situated between the first identifier bit and a first bit of thedata length code.
 45. The method as recited in claim 44, wherein atleast one additional status bit is provided through which theapplication software is provided with information relating to the datatransmission method deviating from CAN standard ISO 11898-1.
 46. Themethod as recited in claim 45, wherein the at least one additionalstatus bit includes at least one of: i) a status bit for communicatingsuccessful transmission, ii) a status bit for communicating successfulreception, and iii) at least one status bit for communicating a type ofthe error that most recently occurred.
 47. The method as recited inclaim 46, wherein as a function of the frequency of occurrences oferrors in a data transmission method deviating from CAN standard ISO11898-1, a changeover takes place back to the transmission methodaccording to CAN standard ISO 11898-1, and at least one status bit isprovided for the communication of the change back that has taken place.48. The method as recited in claim 47, wherein in at least one of; i)the at least one buffer memory, and ii) at least one associated messagememory, at least one additional message bit is provided that identifiesthe method of data transmission to use for the respective message. 49.The method as recited in claim 48, wherein the at least one additionalmessage bit corresponds to at least one of the values of the firstidentifier or further identifier or is derived from the values thereof,or corresponds to at least one of the values of bits r1 or r0 of thecontrol field or SRR of the arbitration field.
 50. A device for serialdata transmission in a bus system having at least two participating dataprocessing units that exchange messages via the bus, the sent messageshaving a logical structure in accordance with CAN standard ISO 11898-1,the logical structure including a start-of-frame bit, an arbitrationfield, a control field, a data field, a CRC field, an acknowledge field,and an end-of-frame sequence, the control field including a data lengthcode that contains an item of information about the length of the datafield, wherein given the presence of a first changeover condition,deviating from CAN standard ISO 11898-1 the data field of the messagescan include more than eight bytes, wherein the device is configured tointerpret values of the data length code, when the first changeovercondition is present, at least partly in a manner deviating from CANstandard ISO 11898-1 to determine the size of the data field, whereinthe device includes at least one buffer memory for forwarding of databetween the data field and application software, and wherein the deviceis configured to adapt a quantity of data to be forwarded to theapplication software in a manner at least corresponding to a differencein size between the data field and the buffer memory used if the size ofthe data field differs from the size of the buffer memory used.
 51. Thedevice as recited in claim 50, wherein the device is configured toselect from the data field of a message received via the bus, a quantitycorresponding to the size of the memory buffer according to a detectionmethod, and to forward the quantity of data to the buffer memory of thesize of the data field is greater than the size of the buffer memory.52. The device as recited in claim 51, wherein the device furtherincludes at least one protocol control unit to adapt the quantity ofdata forwarded between the data field and the buffer memory in a mannercorresponding to the difference in size between the data field and thebuffer memory.
 53. The device as recited in claim 52, wherein the deviceincludes at least one additional or expanded status register whosecontent identifies at least one of: i) a type, ii) a success, and iii) aresult, of the respectively used data transmission method.
 54. Thedevice as recited in claim 53, wherein the device includes at least oneof: i) an additional message memory, ii) an expanded message memory, andiii) a buffer memory, in which at least one additional message bitidentifies the method of data transmission that is used for therespective message.
 55. A method for serial data transmission in a bussystem of a motor vehicle having at least two control devices thatexchange messages, via the bus, sent messages having a logical structurein accordance with CAN standard ISO 11898-1, the logical structureincluding a start-of-frame bit, an arbitration field, a control field, adata field, a CRC field, an acknowledge field, and an end-of-framesequence, the control field including a data length code that containsan item of information about the length of the data field, wherein giventhe presence of a first changeover condition, deviating from CANstandard ISO 11898-1, the data field of the messages can include morethan eight bytes, at least one buffer memory being provided forforwarding of data between the data field and application software, themethod comprising: interpreting values of the data length code, at leastpartly in a manner deviating from CAN standard ISO 11898-1, to determinea size of the data field, when the first changeover condition ispresent; and if the size of the data field differs from the size of thebuffer memory, adapting the data to be forwarded to the applicationsoftware in a manner at least corresponding to the difference in sizebetween the data field and the buffer memory.
 56. A method for serialdata transmission in a bus system of an industrial installation havingat least two control devices that exchange messages, via the bus, sentmessages having a logical structure in accordance with CAN standard ISO11898-1, the logical structure including a start-of-frame bit, anarbitration field, a control field, a data field, a CRC field, anacknowledge field, and an end-of-frame sequence, the control fieldincluding a data length code that contains an item of information aboutthe length of the data field, wherein given the presence of a firstchangeover condition, deviating from CAN standard ISO 11898-1, the datafield of the messages can include more than eight bytes, at least onebuffer memory being provided for forwarding of data between the datafield and application software, the method comprising: interpretingvalues of the data length code, at least partly in a manner deviatingfrom CAN standard ISO 11898-1, to determine a size of the data field,when the first changeover condition is present; and if the size of thedata field differs from the size of the buffer memory, adapting the datato be forwarded to the application software in a manner at leastcorresponding to the difference in size between the data field and thebuffer memory.
 57. A method for serial data transmission in a bus systemof a motor vehicle, the bus system including a control device of themotor vehicle and a programming unit connected to the bus system forprogramming during manufacturing or maintenance of the motor vehicle,the control device and the programming unit exchanging messages via thebus, sent messages having a logical structure in accordance with CANstandard ISO 11898-1, the logical structure including a start-of-framebit, an arbitration field, a control field, a data field, a CRC field,an acknowledge field, and an end-of-frame sequence, the control fieldincluding a data length code that contains an item of information aboutthe length of the data field, wherein given the presence of a firstchangeover condition, deviating from CAN standard ISO 11898-1, the datafield of the messages can include more than eight bytes, at least onebuffer memory being provided for forwarding of data between the datafield and application software, the method comprising: interpretingvalues of the data length code, at least partly in a manner deviatingfrom CAN standard ISO 11898-1, to determine a size of the data field,when the first changeover condition is present; and if the size of thedata field differs from the size of the buffer memory, adapting the datato be forwarded to the application software in a manner at leastcorresponding to the difference in size between the data field and thebuffer memory.
 58. A method for serial data transmission in a bus systemof an industrial installation, the bus system including a control deviceof the industrial installation and a programming unit connected to thebus system for programming during manufacturing or maintenance of theindustrial installation, the control device and the programming unitexchanging messages via the bus, sent messages having a logicalstructure in accordance with CAN standard ISO 11898-1, the logicalstructure including a start-of-frame bit, an arbitration field, acontrol field, a data field, a CRC field, an acknowledge field, and anend-of-frame sequence, the control field including a data length codethat contains an item of information about the length of the data field,wherein given the presence of a first changeover condition, deviatingfrom CAN standard ISO 11898-1, the data field of the messages caninclude more than eight bytes, at least one buffer memory being providedfor forwarding of data between the data field and application software,the method comprising: interpreting values of the data length code, atleast partly in a manner deviating from CAN standard ISO 11898-1, todetermine a size of the data field, when the first changeover conditionis present; and if the size of the data field differs from the size ofthe buffer memory, adapting the data to be forwarded to the applicationsoftware in a manner at least corresponding to the difference in sizebetween the data field and the buffer memory.